I can't remember his name, but in what I consider the WORST CAMPAIGN EVER, the DM had a Mary Sue that just ticked me off (though, to be truthful, the entire session was the worst game of D&D I've ever played and caused the break-up of my original group, so complaining about one guy isn't much).
He was a quickling/god of sorts that interrupted our combats and defeated enemies, took all of our weapons when we, as players, were about to fight eachother, could traverse any distance or location instantly, even passing through traps that we couldn't.. The irritating part was he was played with this squeaky voice and there was never a backstory on the guy (at least in that session).
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But the worst one. The one that takes the taco has to be Aloyicious. He was a "merchant" of questionable morals in a campaign back in 3e. He funded all our adventures and was quite antagonistic towards us. Nothing we did could harm or irritate him, and we were forced to just do whatever the DM made us do, through him.
No matter where we went (including a massive travel across the continents) he was always there. Around level 6 we were fighting off some drow or something when an ancient wyrm black dragon appeared to destroy us for plot-points we couldn't understand. Before he could attack, Aloyicious showed up said, "no" and "wished him out of existence"..I don't get it, either. Turns out, Aloyicious was a pit fiend in disguise using us to do his bidding. He was angry at having to "save us" and thus killed my character and the party barbarian out of spite.
He was some sort of "character" from a previous planescape game the DM played in with his older group. The character was trying to win the blood war and we were his pawns. Nevermind that nothing we ever actually did served to forward that goal, they were just odd little things that meant nothing. And, the fact that he killed half the party when irritated...
It was a stupid campaign that pretty much ended after that; it lasted 2-3 more sessions before we dumped it in favor of playing anything else.